This invention relates generally to magnetic resonance imaging and more particularly the invention relates to continuous fluoroscopic imaging using spiral k-space scanning.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.0, of the precession of the nuclei is the product of the magnetic field B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =Z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially-localize the FID signals in the plane. The angle of nuclear spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
A k-space interpretation of nuclei excitation is given by Pauly, Nishimura, and Macovski in "A k-space Analysis of Small-Tip-Angle Excitation," Journal of Magnetic Resonance 81, 43-56 (1989).
As is well-known in the art, the read-out of magnetic resonance signals can be formulated as a sampling in the Fourier spatial-frequency domain, or a k-space trajectory including spirals emanating outwardly in the frequency domain. See Twieg, "The k-Trajectory Formulation of the NMR Imaging Process with Applications in Analysis and Synthesis of Imaging Methods," Medical Physics 10(5), pp. 610-621, September/October 1983, and Ljunggren, "A Simple Graphical Representation of Fourier-Based Imaging Methods," Journal of Magnetic Resonance 54, pp. 338-343, 1983.
Spiral k-space scanning is particularly well suited to continuous, fluoroscopic acquisition. Recent developments in functional, interventional, and kinematic MRI suggest possible new applications for MR fluoroscopy. Heretofore, single-shot echo planar imaging has been used in a fluoroscopic mode with excellent time resolution, but high spatial resolution is not possible on a standard imager. Early MR fluoroscopy studies on standard imagers focused on 2DFT acquisition; the real-time capability of the systems were impressive, but the image sequences suffered from discontinuities when the raster scan passed through the origin in k-space. More recently, there have been attempts to reduce the discontinuities in 2DFT fluoroscopy by either acquiring the center of k-space more often than the edges or by only acquiring the edges of k-space at the start of the scan. As disclosed by Meyer et al. "Fast Spiral Coronary Artery Imaging" Magnetic Resonance in Medicine, 28:202-213 (1992), interleaved spiral scanning works well in a standard imager because a portion of the center of k-space is acquired during each interleaf, so that abrupt updates of the low spatial frequencies are avoided,
The present invention is directed toward improved fluoroscopic magnetic resonance imaging using spiral k-space scanning.